Observations and Modeling of Shear Reduction and Sediment Flux within Vegetated Canopies on Managed Coastal Dunes
Thesis Abstract: Wind flow on vegetated coastal fore dunes adapts to the local canopy drag, resulting in spatial gradients in bed shear stresses which contribute to the formation of localized bedforms (e.g., nebkha, shadow dunes). Numerous morphological properties of the plants, including canopy height and density, affect the wind flow dynamics and therefore influence depositional patterns within the dune complex. For example, in a field and laboratory study using vegetation proxies, Hesp et al. (2019) found that there is a threshold canopy density ( 40% surface cover) below which there is an adjustment length from the leading edge of the canopy to the initial point of sediment deposition. In managed coastal dunes, vegetation is commonly manipulated for a variety of stakeholder-driven needs (e.g., restoration, sediment stabilization, and grading), with managed dunes often having vegetative densities that are significantly lower than their natural counterparts for numerous years post-management. Understanding sediment transport pathways and the distribution of sediment deposition within sparsely vegetated dune complexes is critical for quantifying the ecological, economic, and protective benefits of dune management activities. However, limited tools currently exist for simulating ecomorphodynamic evolution of managed dune systems. Here we present a combined field and numerical modeling study to quantify the adjustment length scale and its importance to account for numerically simulating landscape changes within sparsely vegetated dune canopies in managed sites. We first present field observations of the near-bed wind profile and sediment transport field from a recently planted dune stabilization project in Pacific City, OR, USA (10% surface cover planted 3 months previous) and Bayshore, OR (30% surface cover, planted 2 years previous). Measurements were collected at incremental downwind distances from the leading edge of the canopy to capture the influence of the adjustment length in a field setting. Field-derived parameterizations of the adjustment length are combined with the shear coupling method of Okin (2008) to develop a new approach to account for the bed shear stress field in the lee of individual dune stands. This new vegetation-wind interaction formulation is implemented into Aeolis (Hoonhout and de Vries, 2016), a process-based, multifraction aeolian sediment transport model, to test its implications for wind-driven sediment flux. A comprehensive comparison of this spatially dependent shear reduction, which incorporates an adjustment length scale, to the widely used grid-based shear coupling approach of Raupach et al. (1993) is also presented. Our results suggest that the modeled morphologic trajectory of incipient dune growth within a newly planted canopy varies significantly with the choice of shear coupler and the inclusion of the adjustment length term. The choice of shear coupler is most important for areas characterized by sparse vegetation such as proto dunes experiencing incipient vegetation colonization or dunes planted after grading or for surface stabilization, with the two shear coupling approaches converging to similar rates of sediment trapping for dense vegetation.